This invention relates to the manufacture of electrochemical cells and, more specifically, to an electrochemical cell and a method for making such a cell, in which an insulator is disposed at the interior of the cell casing between exposable portions of an electrode plate assembly and the conductive casing.
The typical electrochemical cell is of cylindrical form, the familiar pen-light or flashlight size battery, for example, in which the cell comprises a conductive cylindrical casing which serves as the negative terminal for the cell and an internal spirally wound plate electrode assembly of interleaved positive and negative electrodes. During manufacture, the coiled electrode assembly is inserted into the open end of the cylindrical casing. An electrical connection is made between the positive electrode plate edges and the central terminal of an insulated top closure member. Thereafter, the closure member is inserted into the top of the casing and is secured to its rim in the conventional manner such that the electrically positive cover assembly is insulated from the electrically negative casing.
It has been found with electrochemical cells constructed in this manner that the coiled plate electrodes can telescope, i.e., displace axially of the cell, relative to one another, particularly at the bottom of the casing. If the displacement of the positive plate convolutions is excessive, they will make electrical contact with the bottom of the casing. Since the casing is electrically negative, the cell becomes shorted out if any of the edges of the positive plates have touched the bottom of the casing. For this reason, it is sometimes required to use an insulator at the bottom of the casing between the casing surface and the lower edges of the coiled electrode plate assembly. For example, a thin polyethylene disk can be placed in the bottom of the casing prior to insertion of the coiled plate assembly to insulate the electrodes from the casing. Although this does solve the problem of inadvertent shorting of the cell due to the electrode telescoping problem, it poses manufacturing disadvantages.
It is the usual practice to stamp out the cylindrical casings separately and to feed these casings from a hopper or bin during subsequent manufacturing steps, for example inserting the coiled plate electrode assembly into the casing. If an attempt is made to place the insulating disk at the bottom of the casing prior to the time that they are loaded into the hopper, the insulator disks can become displaced or fall out altogether. Moreover, in some manufacturing operations, the coiled plate assembly is inserted into the casing can while the latter is oriented horizontally. Any attempt to put the insulator disk into the bottom of the casing even immediately prior to insertion of the plate electrode assembly, makes it difficult to keep it in place.
Another alternative approach--the use of adhesive insulators which would stick in place once inserted into the casing--is not a practicable solution because of the difficulty in handling materials which are adhesive-coated. That is, an adhesive backing on the insulator would preclude rapid feeding and placing of the insulators into the casings on a production line basis because of the tendency of the adhesive backing to adhere to other insulators and machine components.